1. Technical Field
This invention relates to a distributorless ignition system for internal combustion engines, and more particularly, to an inductive discharge ignition system.
2Description of Prior Art
Recently, higher ignition effect and efficiency are required from an ignition system to gain higher power and better fuel combustion from an engine of a vehicle. U.S. Pat. No. 4,462,380 (hereinafter '380 patent) issued to J. R. Asik, discloses a distributorless ignition system that uses a supplementary spark energy (SSE) module to increase ignition energy. FIG. 8A herebelow shows a block diagram of a distributorless ignition system for internal combustion engines according to the '380 patent. Two ignition coils T1 and T2 have primary windings PR1 and PR2 which are driven by ignition modules No.1 and No.2 correspondingly. Secondary windings SEC of the ignition coils T1 and T2 are coupled in series with spark plug electrodes and the SSE module. The '380 patent teaches employing a simple DC to DC converter as the SSE module. FIG. 8C shows a schematic diagram of the SSE module that converts 12 V DC to 3 kV DC. Referring to FIG. 8B for explaining the manner of the circuit, the I.sub.s-p current in spark plugs (SP-1, SP-2, SP-3 and SP-4) changes according to following expression (t is the time, L is inductance, V.sub.s-p is constant spark holding voltage between spark plug electrodes in a `post-break-down` phase, and V.sub.em is a constant output voltage of the SSE module): ##EQU1## V.sub.s-p depends proportionally on the distance between spark plug electrodes and may differ significantly for different cylinders of an engine. FIG. 8B shows the I.sub.s-p current in a spark plug with the SSE module (line I) and without it (dotted line II). The SSE module provides for an extension of ignition time that increases ignition energy of each stroke. Ignition time may be represented as follows: ##EQU2##
Nevertheless a need still remains for reduction of rated power and miniaturization of ignition coils, especially when said coils are disposed directly on spark plug heads. In the above mentioned ignition system reduction of inductance L (which results in reduction of energy that can be stored in the ignition coils T1 and T2) causes shortage of the ignition time T.sub.i (see expression 2). This reduction cannot be compensated for by increasing of the output V.sub.em voltage of the SSE module. As mentioned above, the spark holding V.sub.s-p voltage corresponds to spark plug gap as well as to pressure, temperature and other parameters of the fuel/air mixture in a cylinder. Because all of these parameters have tolerances , V.sub.em must be kept much lower than mean V.sub.s-p (see expression 1). Otherwise, the spark plug current I.sub.s-p can be excessive and, in the extreme case, continuous and uncontrollable. The use of a dummy load diminishes this defect, but is not able to eliminate it. In that way the reduction of inductive energy in above mentioned system leads to ignition unstability.
Enhancing of ignition power may be also achieved by means of high frequency sparking that is set forth in U.S. Pat. No. 4,938,200 issued to S. Iwasaki. This patent discloses a relatively high frequency ignition device which comprises a basic ignition coil for all spark plugs, and a high voltage transformer for each spark plug. This transformer is magnetized periodically in one direction for several times during an ignition interval. The first magnetization pulse should have a duration that is sufficient to ensure ignition. Therefore, transformer dimensions cannot be reduced significantly.
Other related patents, employing relatively high frequency ignition devices, include U.S. Pat. No. 4,326,493 issued to J. Merrick, and U.S. Pat. No. 4,947,821 issued to M. Somiya. The structure of high frequency ignition devices is complex and supposes use of step up DC to DC converters to reduce time for energy accumulation in inductors and capacitors.
U.S. Pat. No. 4,892,073 issued to N. Yamamoto et al, discloses a conventional ignition system comprising individual ignition coils for each spark plug. As is shown by John B. Heywood in his `Internal Combustion Engine Fundamentals`, McGraw Hill, Inc., 1988, FIG. 9-39, for a conventional coil spark ignition system, an ignition coil in this system is demagnetized in wide range of intensity, that is, the voltage on spark plug electrodes jumps up to 15-20 kV during a few microseconds in a `pre-break-down` phase but remains respectively low (0.5 kV) in a `post-break-down` phase, during 1.5-2.0 milliseconds. Because ignition coils with laminated iron cores have more capacity for accumulation of energy in magnetic field than coils with other core materials, they are still commonly used. But use of these laminated cores results in high eddy currents in the `pre-break-down` phase as the voltage on the secondary winding of the coil corresponds to the high voltage on spark plug electrodes. These eddy currents decelerate the `pre-break-down` phase and reduce available voltage of an ignition coil. And vice versa, ferrite cores are able to provide an effective jump of voltage in the `pre-break-down` phase, but they are unable to store comparable quantity of energy in the same volume to efficiently keep ignition process going in the `post-break-down` phase, so there is still a need for improved disributorless ignition systems, that are free from the disadvantages described above.